Toothsome … The Gruffalo as seen in the 2009 BBC adaptation. Photograph: BBC/Magic Light Company

In one episode of much-loved sitcom Father Ted, the young priest Dougal confesses that the spider-baby he saw on TV was actually something he’d dreamed. Ted shows him a diagram of a man’s head. Inside it is the word Dreams. Outside, is the word Reality. “Have you been studying this like I told you?” Ted asks.

For the mouse protagonist of Julia Donaldson and Axel Scheffler’s bestselling picture book The Gruffalo, no such convenient distinction exists between the outer and inner worlds. To fend off a series of murderous, though beguiling, invitations to dinner from ravenous predators during his stroll through the dark forest, the trickster mouse invents a previous invitation with an imaginary friend.

All goes well in the scary psychological forest of childhood as, one by one, the would-be devourers leave in fright. Until it all goes horribly wrong: at midpoint, the fearsome Gruffalo is physically conjured. “Oh help! Oh no! It’s a Gruffalo,” bleats the hitherto cocky smartarse mouse. “You’ll taste good on a slice of bread,” replies the beast – about the only honest statement in the story.

It’s one that brims with echoes. Beware of what you wish for. Don’t cry wolf. The golem servant-menace of Jewish folklore. And poor Victor Frankenstein, in Mary Shelley’s novel, rushing out in “breathless horror and disgust” after seeing “the dull yellow eye of the creature open”.

The Gruffalo is comic horror. But while echoing other tales and genres, the story stands alone. It features a unique and unsurpassed monster: what you imagine can kill you. It’s bizarre to think he’s only been around since 1999, so culturally embedded is he now.

Just as he is anatomically constructed by the mouse from disparate elements, like some hellish wishlist for a genetically modified organism, the Gruffalo’s mythic components are similarly eclectic. To start with, he’s not summoned by grave-robber necro-medicine, but by incantation:

“His eyes are orange, his tongue is black;He has purple prickles all over his back.” [...]“He has terrible tusk, and terrible clawsAnd terrible teeth in his terrible jaws.”

We are in magic-land; the mouse is tempting the evil eye. He could be saying Candyman in front of the mirror or dreaming A Nightmare on Elm Street’s Freddy Krueger – the nightmare terrifyingly made flesh.

Yet the Gruffalo differs fundamentally from such monsters. The mouse’s ghastly gothic Lego behemoth is originally purely mental – an act of devious imagining. So, unlike the Candyman and Freddy, bent on vengeance after vigilante lynch-mob deaths, the Gruffalo – zoologically bull, bear, warthog and ogre – is both real and newborn, he has no independent backstory.

His first appearance is as a visually shocking composite of coincidences: he has the identical teeth, tusks, orange eyes, turned-out toes, and so on as the mouse has described in his bully-duping fantasy. He’s more or less the devil, complete with horns. His existence cannot be doubted – as Fox, Owl, Snake and more than a generation of infants can testify – but we are never not aware that he has been invented. A delicious frisson of doubt lives on every page where he appears.

That makes the Gruffalo far more than a children’s book bad guy, or even Old Nick. He’s a universal dilemma at the heart of philosophical questioning: do we construct what we see? What is real?

He enacts, too, in his lurid comedy skin, the infantile fantasy of omnipotence. The mouse dreams a world, and lo, it is so. As the doppelganger-mouse-Gruffalo, the mouse is invulnerable, provided he can survive his own creation. What child, when feeling alone and very little, wouldn’t want some of that invincibility?

All superbeings, from Ninja Turtles to X-Men and Terminators, protect because they can destroy. Happily for the mouse, the Gruffalo, like the conventional superheroes, is flawed.

His first big mistake is to delay gratification. He’s more curious to see whether the mouse really is, as it claims, “the scariest creature in this wood”, than in turning him into fast fingerfood. Opening himself up to the rodent’s ruse gives the Gruffalo likeability. Just as we might have wanted to tell the mouse not to tempt the evil eye by summoning the Gruffalo, now we might wish to tell the Gruffalo not to be taken in by the mouse.

So the Gruffalo becomes a hulking menace, a malevolent Jungian shadow-self, as it follows the mouse through its second encounters with Fox, Owl, and Snake. These killers, unbeknown to the Gruffalo, whose eyes fix hypnotically on the mouse, scarper when they see him. Thus the Gruffalo, once materialised, saves the mouse a second time.

The mouse’s need is immense, it’s life or death. The forest in which he chooses, perhaps perversely, to stroll, while other small creatures keep their camouflaged distances – a woodpecker, squirrel and frog – is an utterly loveless place. There will be no homecoming to a cosy house and still-warm supper (Where the Wild Things Are); no forgiveness for the risks endured (Harry the Dirty Dog); no family restaurant supper after The Tiger Who Came to Tea has scarfed the entire larder. There is only the constant threat of death and the necessity of living on one’s wits.

In the penultimate scene, the mouse says to his astonished alter ego, “You see? Everyone is afraid of me!” - and makes a low sadistic jibe about liking “Gruffalo crumble”. Poor, duped Gruffalo runs “as quick as the wind”, with ears folded back like a frightened dog.

The moral at the end of the quest, as the mouse finally eats a nut in peace? What you imagine can kill you, but, with enough guile in a deceitful world, it can become your unwitting protector Always keep a Gruffalo in your mind’s back pocket and hold on for the ride.